Field of the Invention
The present invention relates to an image forming apparatus and control method therefor.
Description of the Related Art
An image forming apparatus is generally known to change the quality of an output image formed on a printing medium upon a change of environmental conditions such as the temperature and humidity. An electrophotographic image forming apparatus is known to change the quality of an output image along with deterioration of consumables including a photosensitive drum, toner cartridge, and toner, and the like.
Coping with the change of the quality of an output image and adjusting the quality to a desired characteristic require calibration for adjusting image forming conditions. For example, Japanese Patent Laid-Open No. 2000-238341 discloses a method of generating a lookup table (LUT) for performing density correction or tone correction (γ correction) by reading a test pattern image formed on printing paper. According to this method, a reference density value is determined by measuring the density of a toner image formed on a photosensitive drum in accordance with a generated LUT. The LUT is then corrected based on the result of a comparison between the reference density value and a density value obtained by measuring a toner image formed again on the photosensitive drum at a predetermined time. The stability of image density characteristics can therefore be maintained for a long term.
Japanese Patent Laid-Open No. 09-146313 discloses a method of executing, as calibrations in an image forming apparatus, maximum density correction and γ correction every time printing is done for a predetermined number of sheets, and also executing line width control to adjust the line width by changing the laser exposure amount. According to this method, γ correction is executed after performing line width control and maximum density correction in order not to change, by line width control and maximum density correction, density characteristics corrected by γ correction. A light-emitting element and light-receiving element are arranged to detect a line image or the like formed on an image carrier for line width adjustment. Based on the detection result, line width control is executed.
In γ correction according to Japanese Patent Laid-Open No. 2000-238341, when the density of a toner image formed on a photosensitive drum is higher than the reference density value, the LUT is corrected to decrease a laser output signal value used to perform normal image formation. In this case, especially when a laser output signal value corresponding to high density level is decreased, a character or line may be disconnected in an output image formed on a printing medium depending on the degree of decrease, generating a jaggy. The jaggy of the output image stands out particularly when the image is output using a low-brightness developing material. Although the stability of density characteristics in the output image can be maintained, the quality of the output image may degrade due to degradation of the line quality. To reduce the degradation of the quality of the output image, it is desirable to appropriately adjust the line width.
However, line width control according to the method in Japanese Patent Laid-Open No. 09-146313 requires a light-emitting element and light-receiving element to detect an adjustment test pattern image formed on the image carrier. Hence, additional hardware and processing are needed.